This invention relates to torque control of a sinusoidally excited permanent magnet motor, and more particularly to reducing the low frequency torque ripple, or smoothing the torque, in such a motor.
It is known in the art relating to electric motors that polyphase permanent magnet (PM) brushless motors with a sinusoidal field offer the capability of providing low torque ripple, noise, and vibration in comparison with those with a trapezoidal field. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back-emfs (also known as xe2x80x9cback-voltagesxe2x80x9d), the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from those assumptions of pure sinusoidal back-emf and current waveforms. The deviations will usually result in parasitic torque ripple components at various frequencies and magnitudes. The methods of torque control can influence the level of this parasitic torque ripple.
One method for torque control of a permanent magnet motor with a sinusoidal back-emf is accomplished by controlling the motor phase currents so that its current vector is aligned with the back-emf. This control method is known as the current mode control method. In such a method, the motor torque is proportional to the magnitude of the current. The current mode control method requires a complex controller for digital implementation. The controller requires two or more A/D channels to digitize the current feedback from current sensors. In a three-phase system, it is convenient to transform the three-phase variables into a two dimensional d-q synchronous frame which is attached to the rotor and design the controller in the d-q frame. But, due to considerable calculations and signal processing involved in performing the d-q transformation, reverse d-q transformation and P-I loop algorithms, a high speed processor such as a digital signal processor (DSP) has to be used to update the controller information every data sampling cycle.
A method and system for controlling the torque of a sinusoidally excited PM motor to reduce low frequency torque ripple, or smooth torque, is disclosed. The low frequency torque ripple is reduced by a controller which calculates the voltage required for producing the desired torque based on motor equations. The controller is implemented using only feedback of the rotor position and speed.
The method and system of the invention preserve the smoothness of sinusoidal commutation while eliminating the sensitivity of torque ripple due to current sensors of the prior art. The controller of the invention features a low cost implementation that not only eliminates the hardware of current sensors and A/D converters of the prior art, but also considerably reduces the software computation needs, e.g., no d-q transformations and P-I loops are necessary. A low cost microprocessor may be used with the invention instead of the DSPs of the prior art.
The method of the invention senses angular positions of a rotor and determines its rotational speed. In response to the position and speed of the rotor and a torque command signal, a controller develops varying motor voltage command signals indicative of the voltage needed to produce a desired motor torque. Phase voltages are applied across the motor windings in response to the motor voltage command signals to develop the desired motor torque.
The system disclosed herein includes a rotor position encoder coupled to the motor for sensing the angular positions of the rotor and outputting a position signal. A speed measuring circuit is connected to the position encoder for determining the speed of the rotor and outputting a speed signal. The position and speed signals are applied to a controller. The controller develops varying motor voltage command signals in response to the position signal, speed signal, and a torque command signal indicative of a desired motor torque. A power circuit is coupled between a power source and the controller for applying phase voltages across the motor in response to the motor voltage command signals to develop the desired motor torque.
One application for the voltage mode control method and system disclosed herein is in a power steering controller for an electric power steering system. The motor is coupled directly into the steering column to provide steering assist torque. Therefore, even a small level of low frequency torque ripple produced by the motor can be felt at the steering wheel. By using the voltage mode control method disclosed herein, the low frequency torque ripple is reduced and a smooth steering feel is achieved.